Grinding plants are used to comminute bulk material. Often such grinding plants also comprise drying equipment to simultaneously reduce the bulk material's moisture content. Typical examples of such grinding and drying plants are used to process granulated blast furnace slag for the production of cement or so-called coal grinding and drying plants to turn wet coarse raw coal into dry pulverized coal, to be injected into blast furnaces or fired in power plants.
In case the bulk material to be ground and dried is combustible, e.g. in the case of coal, the resulting product is explosive and special attention has to be paid to the design of the process and of the plant, in order either to prevent/avoid explosions, primarily by keeping the oxygen concentration in the gases in contact with the explosive material below the so-called lower explosion limit value (explosion preventing design) or to protect the equipment and the environment against the effects of such explosions (explosion protecting design).
In typical grinding and drying plants, comminution, normally grinding, and drying of the raw material is performed, largely in parallel, inside a comminuting equipment or mill. Coarse material is ground e.g. between rotating rollers, balls etc., and a rotating grinding table or bowl, and moisture is evaporated in contact with a hot drying gas. The drying gas conveys the ground material into a classifier, usually integrated into the top part of the mill. Coarse material is eliminated from the drying gas flow and returned onto the grinding table or bowl, the fine material is transported by the cooled-down waste drying gas with increased water vapor content into a downstream equipment for gas-solid-separation, usually a bag filter.
Although many improvements have been made so far in the conception and operation of such grinding and drying plants, it remains that the overall process is very cost intensive in terms of energy consumption.